Hi Carerra,
I'm really glad to hear you are finding Quick Base accessible while you are getting to know your app and planning your future changes. For your question this is a way to create a button to add a Reminder from many places and have it tie back to that Client record. By default when you create a relationship a new button is made to help you create child records from the parent. If you go into the field properties for that button you will typically see something like:
URLRoot() & "db/" & [_DBID_WORKORDERS] & "?a=API_GenAddRecordForm&_fid_22=" & URLEncode ([Record ID#])& "&z=" & Rurl()
To break it down what this button is doing is saying take me to my child table (in this case Work Orders) and open up a form and bring over the parent records Record ID and put it into the Related Parent field (field id 22) in my child table. This way when you go to the reminder record the relationship is already set up for you. Since you are starting in the middle by adding from Opportunities the pre made button is grabbing your Related Opportunity but not your Related Client information.
Looking at the field setting for your Add Reminder button you can actually go in and edit it to have it also grab the Related Parent for your Opportunity and pass it down to the related parent field on your Reminders table after you get a little information. For that you want to know the Field ID for the Related Client field on your Reminders (You can find this by opening up the Reminders table, finding the Related Client field on the Reminder table, and writing down its field id that number that shows up in the field settings). Then you need to know the name of your Related Client field in your Opportunities (Most likely it will be Related Client) from there you can edit the button to capture that information.
For my example lets say that the Related Client field in my Workorders table is 21 and the Related Client field in my Opportunities is Related Client. I would alter the button like below:
URLRoot() & "db/" & [_DBID_WORKORDERS] & "?a=API_GenAddRecordForm&_fid_22="
& URLEncode ([Record ID#])& "&_fid_41=" & [Related Client] & "&z=" & Rurl()
What I have done is add on to that button to include "&_fid_41=" & [Related Client] & which tell its to grab the value from Related Client on my Opportunities table and put it into my Related Client field on my new Reminder I am going to create. You just need to take your existing buttons and edit them to include that extra piece of information. If you aren't feeling up to making that change on your own you can also always open up a support case with a Care Team to help walk you through how you would update one of your buttons. I hope this information is helpful.
